I am using a mat foundation job using STAAD.Foundation 5.3. Where can I enter the density of concrete ?

In version 05.03. there is no option to directly define the concrete density. However the software internally uses a value of about 150 lb/ft3 as the density for concrete. If needed, you can modify the dead weight factor which you can access through the Add Selfweight and Deadweight Factor table. This table can be accessed from the toolbar region at the top as shown next.

In more recent versions of STAAD Foundation Advanced like version 7, the concrete density can be directly defined from within Mat Foundation Job > Default Properties option within the Main Navigator. There is an input called Concrete Property within the Data Input Pane on the right, where concrete density can now be entered.